HOW: Lie on your back. Place a band (if you have one) around your knees. Keeping your stomach tight, squeeze both of your glutes at the same time and attempt to spread your knees apart as far as you can.
FEEL: You should feel both of your glutes working to push your knees apart.
COMPENSATION: Avoid arching your back as you perform this. Keep the motion isolated to the hip as best you can.